The present invention relates to an improved truck spring suspension system and to bushings which may be used therein either to attach a spring shackle to a truck chassis or to attach a leaf spring to a truck chassis bracket or a spring shackle.
Heavy trucks must have suspension systems of sufficient strength and stiffness to support the vehicle and its cargo under whatever highway conditions they might encounter. Historically suspension systems satisfying these requirements have not satisfactorily attenuated roadway induced shock and vibration. A very stiff suspension system reduces the usable life of a vehicle because roadway induced shock and vibration loads are not isolated from the truck's critical components; if a suspension system is not designed properly or does not function as designed the input loads can be greatly amplified by the truck frame and other structure, including an overly stiff suspension system. Conversely, a more flexible suspension system acts to isolate roadway inputs from a truck's critical components. More importantly, not only is the fatigue life of a truck itself reduced by an overly stiff suspension system, the truck's driver is also subjected to vibratory and shock environments which result in his premature fatigue and exhaustion. Even more serious than the driver's temporary discomfort is the possibility of an accident resulting from the driver's fatigue. A still further concern is long-term damage to the driver's health.
The truck manufacturing industry has produced relatively good smooth-riding truck suspensions when the truck is new with a combination of tires, springs, air-cushion suspensions and other types of shock absorbers. The problem is that such smooth-riding characteristics often are lost quite early in the life of the truck, e.g., 50,000 miles or less. The present invention is directed to a way to maintain the operational characteristics of a properly functioning leaf spring suspension system after extended periods of use and even after missing a spring pin and bushing lubrication cycle. Also, the present invention achieves the result of requiring minimal design and manufacturing changes for original equipment applications as well as being particularly well suited to retrofit applications. Retrofit is very economical and requires no significant structural alterations. Installing the present invention into a suspension system having prior art bushings is not much more difficult than replacing the existing bushings.
Prior art leaf spring suspension systems have used brass or other similar solid-section type bushings to allow oscillatory motion of the spring pivot points. These bushings, however, were susceptible to accelerated wear and exhibited binding behavior if they were not frequently lubricated. Recommended lubrication intervals for solid-section type bushings in heavy truck suspension applications are about every 2500 miles. However, such suspensions actually get lubricated no more frequently than about every 5000 miles. Once the bushing is "run dry" the ostensibly pivoting connection rapidly degrades in performance. The spring ends are not as free to rotate as they would be under ideal conditions. At this point the spring ends move from semi-simply-supported to semi-cantilevered boundary conditions and the undesirable manifestations of a fixed-end spring result.
The present invention is directed to producing a bushing assembly that solves the spring bushing freeze-up problem. The invention provides a spring bushing requiring less scheduled maintenance while providing a smoother ride.
Bearings have not previously been used in spring shackle applications because of a number of significant problems, which are solved by the present invention. Loads transmitted through spring bushings in heavy truck suspensions are quite high. Bushing loads can be 3,000 lbs. or higher under simple static loading conditions. Rough roads cause even higher dynamic loads. To withstand such loads, a ball bearing type bushing would be massive, probably 4 to 5 inches in diameter. Without major redesign, current leaf spring suspension systems could not accommodate such large bearings.
To be usable in current systems, a bearing must be compact. However, compact bearings of any type generally require precisely machined contact surfaces. This requirement is at odds with the typical construction and installation of spring shackle bushings. Spring leaf eyes are not machined to close tolerances and can therefore damage a spring shackle bushing sleeve. If such damage occurs, the bearing is subjected to localized frictional loading because of binding, warping, or indentation of the bearing contact surfaces.
A further problem with use of bearings in spring shackle bushings concerns the type of loading they encounter. Whereas bearings do a reasonably good job of distributing high speed rotary loads evenly among all the rolling elements even with loose tolerances, spring shackle bushings experience only oscillatory motion caused by essentially vertical input loads. The unidirectional nature of the input loads combined with loose tolerances causes pounding in a spring shackle bushing. The result is that the load is amplified and unevenly distributed; because of the unidirectional loading some parts or elements carry more than a proportionate share of the total load and, because of the loose tolerances these already highly stressed elements experience an even greater load. The pounding forces due to vertical input loads and loose tolerances result in extremely high stresses in just a few of a bearing's rolling elements.
A theoretical explanation of the functioning of a suspension system having frequency and damping characteristics sufficient to effectively isolate the truck and driver from roadway induced shock and vibration loads will now be given. If the suspension system's natural frequency is sufficiently lower than the fundamental frequencies of the truck and its major components, the suspension system acts as a shock and vibration isolator for the rest of the vehicle. A low frequency isolator works by transforming mechanical energy into thermal energy. At the frequency at which this transformation occurs, the suspension system's natural frequency, the isolator does not reduce the magnitude of the input load (the load is actually amplified). However, the vibration and shock amplification in a properly designed and functioning isolation system occurs at a frequency that does not coincide with the natural frequency of any critical component. Mechanical energy is dissipated at a frequency that does not excite structural resonances of any critical vehicle components. Further, the input load is generally amplified by no more than a factor of three at an isolator resonance, as opposed to much greater amplification at structural resonances in nonisolated systems. At frequencies greater than the isolator natural frequency the isolation system's transmissibility curve "rolls off." That is, the isolator works to reduce the magnitude of input loads at higher frequencies. In other words, at the natural frequencies of the vehicle or its critical components, the effect of isolation is to attenuate shock and vibration loads. The higher frequency loads are actually reduced in amplitude. This desirable result is diametrically opposite to the result in a nonisolated system. In a nonisolated system any input loads are amplified by the structure at the point where the input load frequency coincides with the structure's natural frequency. Amplification of input loads in nonisolated structures similar to trucks is frequently in the 20X (input load magnified by a factor of 20) range, but can be much greater, depending on the inherent hysteresis damping of the particular structure and material.
To effectively attenuate roadway induced shock and vibration loads, a suspension system must be sufficiently flexible and contain sufficient damping properties to either transform the mechanical energy into another form (heat) or store it momentarily for slower release into the vehicle. Effective shock isolation transforms a short duration shock pulse of high amplitude into a longer duration pulse of lower amplitude, the total energy transmitted into the vehicle being the input mechanical energy minus the mechanical energy converted into heat by the isolating medium.
The effect of spring pin freeze-up is to transform an initially pin-ended spring into a cantilevered spring. The result is to increase the natural frequency of the suspension system and thereby reduce the ability of the suspension system to effectively isolate shock and vibration loads from the vehicle and driver. A cantilevered beam of the same cross section and material as a simply supported beam is stiffer and hence is not as proficient at shock and vibration isolation. This is true partly because the natural frequency of a cantilevered beam is higher than for a simply supported beam and hence the attenuating effect at the vehicle structural resonances is not as great (the transmissibility curve has not "rolled off" at the vehicle structural resonances as much as it would have for the lower frequency simply supported condition). But a simply supported leaf spring is also a better isolator because the particular design of a leaf spring allows for relative movement between the spring leaves. This relative movement produces frictional damping. Therefore, the more a leaf spring flexes, the more it damps vibration and shock inputs. Further, low friction pin joints have superior damping properties as compared to rigid, nonpivoting connections, which have no damping propensity beyond whatever internal hysteretic effect might occur.
The undesirable aspects of a fixed-end leaf spring are not confined to the deleterious fatigue effects such springs have on trucks and drivers. For identical input loads, the maximum stress in a cantilevered leaf spring is greater than in the same spring with freely rotating ends. The reason for this difference lies in the particular construction and method of attachment of a leaf spring. The maximum bending moment in a simply supported leaf spring occurs at the midpoint, where the roadway load is transmitted into the spring. However, the maximum bending moment in a fixed-end leaf spring occurs both at the midpoint and at each end. By maintaining free rotation of the spring ends, excessive bending moments at the spring ends are prevented. This is important because in most leaf spring applications the load at each end of the spring is transmitted through only one leaf. Even though the maximum bending moment in a simply supported beam exceeds the maximum bending moment in a fixed-end beam, moving the maximum bending moment to the center of a leaf spring reduces the maximum stress. This occurs because the cross section and hence area moment of inertia is greater at the center than at the ends. Several spring leaves are stacked at the center, and although they are not rigidly connected, the frictional forces between leaves produce a composite moment of inertia exceeding that of a single leaf. Therefore, a simply supported leaf spring is less likely to break at the ends because the effect of free spring end rotation is to move the maximum bending moment to the center, where the spring can better handle the load. Even if the spring does break under excessive loading, in a simply supported spring the failure will most likely occur near the center. However, the failure will not be catastrophic. The initial failure will occur in only a few leaves, giving the driver a warning that total failure is imminent. This scenario is to be contrasted to the situation where spring bushing freeze-up causes breakage near the end. In the latter situation, if the spring is of the type that has only one leaf carrying the total load at the attachment points, catastrophic failure results, including loss of steering and possible rollover.
As can be easily appreciated from the preceding discussion, any device or method for maintaining the flexibility of a leaf spring suspension system will greatly reduce vehicle and driver fatigue as well as the possibility of catastrophic failure of the spring itself. However, even though the aforementioned structural static and dynamic behavior of a leaf spring suspension system was well known, prior attempts to provide a smooth-riding suspension system that was strong and stiff enough to support heavy loads did not solve the problem of spring pin and bushing freeze-up, nor did they provide economical alternative solutions.
A principal aspect of the present invention is to provide an improved spring shackle bushing for heavy truck suspensions and related applications.
A further aspect of the invention is to provide a truck suspension with improved riding characteristics over a substantially longer period of time than obtained with conventional suspensions.
A further aspect of the present invention is to provide an improved spring shackle bushing that reduces frictional forces in leaf spring suspension pivot points.
A further aspect of the present invention is to provide an improved spring shackle bushing that uses roller bearings in combination with very hard and precisely machined bearing contact surfaces to produce low friction leaf spring suspension pivot points.